In-service primary school teachers' professional development and, more specifically, their teacher agency, are analyzed regarding the integration of mathematical education and sustainability. To achieve this, based on semi-structured interviews, several questions involving Education for Sustainable Development (ESD) and links between mathematics education and sustainability are considered, which yielded 44 answers. The analysis of these answers is based on four sub-aspects: knowledge of sustainability and its connection to the SDGs; sustainability practices; links between mathematics education and sustainability; and obstacles and challenges. The results show that teachers exhibit a significant lack of knowledge about sustainability and its connection to the SDGs, making a single association with issues related to the environmental crisis, which is the main focus of the sustainability practices carried out in schools. As pertains to the links between mathematics education and sustainability, most accept the importance of this connection, but point out various obstacles and challenges, such as the lack of knowledge and time, the curriculum itself, and others. It is concluded that it is necessary to design training programs focused on these aspects, in order to contribute to the development of teacher agency, i.e. the appropriation and reconstruction of new resources to face the challenges that mathematics education for sustainability implies in teaching practice.